Cellulose, the most abundant polymer on earth, is a straight-chain polymer of anhydroglucose with beta 1-4 linkages. Cellulose fiber in its natural form comprises such materials as cotton and hemp, while solvent-spun fiber comprises products such as rayon.
The process most frequently used to produce solvent-spun fiber is known as the viscose process. In the viscose process cellulose xanthate, a cellulose derivative, is solubilized then spun into a coagulation bath where the fiber forms. The resulting fiber is a product known as viscose.
A second solvent-spun fiber product, cellulose acetate, is produced when a cellulose derivative is solubilized in an organic solvent, then spun into water or alcohol where it coagulates to form fiber. The fiber may be regenerated from its derivative form to true (nonderivatized) cellulose using an alkaline solution, but such a regeneration step is rarely performed.
In addition to the production of viscose and cellulose acetate, a third embodiment of the solvent-spun process involves the dissolution of true cellulose in a solvent which is then spun into a coagulation bath in which fiber formation occurs.
Due to the high processing costs and the generally inferior properties of the fiber products formed when nonderivatized cellulose is employed in the solvent-spun process, derivatized cellulose such as that used in the viscose process is generally employed when producing solvent-spun fibers.
Approximately 20 years ago it became apparent that the production of solvent-spun fiber by methods such as the viscose process was becoming disadvantageous due to the high capital costs and environmental considerations associated with their use. For this reason, modified or alternative methods for producing solvent-spun fiber were sought.
Several cellulose solvents were tested for use in a modified or alternative solvent-spun process. A few achieved favorable results in solubilizing the cellulose, but were ultimately deemed to be impractical for other reasons.
Specifically, solvents comprising solutions of SO.sub.2 /NH.sub.3, and SO.sub.2 /(CH.sub.3).sub.2 HN were tested and found to form good cellulose solutions (i.e., solutions having reasonable viscosities and practical degrees of polymerization). Unfortunately, it was impractical to regenerate cellulose fiber, and to recover the solvent from the coagulation medium.
Similarly, a 85% H.sub.3 PO.sub.4 solution was tested for use as a cellulose solvent and was found to dissolve cellulose well, however, the resulting solution contained gels and fibers which made filtration very difficult. Additionally, when phosphoric acid was tested, the cellulose went into solution well, but the phosphoric acid could not be washed from the resulting fiber. D. M. MacDonald, The Spinning of Unconventional Cellulose Solutions in Turbak et al., "Cellulose Solvent Systems" ACS Sym. Seri. 58 (1977).
Solutions of 52.5% Ca(CNS).sub.2, DMF/N.sub.2 O.sub.4, and DMSO/para-formaldehyde were also tested. These too proved unsuccessful for use, for while the solutions were found to be acceptable cellulose solvents, they either formed weak fibers or were difficult to recover from the coagulation medium once fiber formation occurred. Hudson, S. M., Cuculo, L. A., J. Macromolecular Science Rev., Macromolecular Chemistry (1980) C18(1) p. 64.
In addition to the solvents listed above, MacDonald (supra) also reported testing a 64% ZnCl.sub.2 solution. As with the previous solvents, the results were unacceptable. In this case, the solubilized cellulose could not be spun and the coagulated fibers were noncohesive.
In view of these results and until the present invention, the use of ZnCl.sub.2 as a cellulose solvent has only been successfully utilized in a limited number of processes.
In one such process, the carbon fiber process, cellulose is solubilized in a ZnCl.sub.2 /HCl solution then extruded into a methanol bath wherein the cellulose coagulates to form fibers. These fibers are usually weak, and while they can generally be handled with tweezers, they are not usually strong enough to permit spinning.
Following coagulation, the ZnCl.sub.2 /HCl is removed from the cellulose by prolonged soaking in the methanol bath. The fibers are then carbonized.
In addition to the carbon fiber process, the production of "vulcanized fiber" or nonwoven mats also involves the use of a ZnCl.sub.2 cellulose solvent. See, eg. Young and Miller, Formation and Properties of Blended Nonwovens Produced by Cellulose-Cellulose Bonding in Gould et al. "Blended Nonwovens," ACS Symp. Ser. 10 (1975).
In producing vulcanized fiber, cellulose is swollen and softened into a gel using a concentrated ZnCl.sub.2 solution. The gel is then pressed into sheets which are leached with water in order to extract the ZnCl.sub.2 from the cellulose. This results in the formation of a tough, rigid, nonwoven plastic sheet.
Prior to the present invention, the use of ZnCl.sub.2 as a cellulose solvent has had limited application as described above. Moreover, its use in a solvent-spun process has until now proven impractical.
The present invention is advantageous, therefore, for it teaches the use of ZnCl.sub.2 as a cellulose solvent in a solvent-spun process, as well as teaching the production of a high tensile strength, solvent-spun cellulose fiber resulting therefrom. The use of ZnCl.sub.2 in the present invention is further beneficial, for ZnCl.sub.2 is nontoxic, less corrosive than previously employed solvents and is easily recoverable for reuse. Further advantages of the present invention are set-forth below.
Absent the teachings of the present invention, it is generally preferred that the cellulose starting material employed in solvent-spun processes have a high degree of polymerization (hereinafter "DP"), i.e. a DP preferably above 600, but at least above 300 (hereinafter "high DP cellulose"), when strong fiber products are desired. Unfortunately, this usually means that the cellulose starting material must be obtained from conventional pulping processes. While the DP of such pulp is normally high, the conventional processes are relatively expensive to operate and their pulp products are generally costly.
Unlike the solvent-spun processes previously discussed, the solvent-spun process of the present invention enables one to use both high DP cellulose and cellulose with a DP ranging from about 100 to 300 (hereinafter "low DP cellulose"). In fact, the cellulose starting material of the present invention may range in DP from about 100 to 3000. Therefore, pulp obtained from a non-conventional, acid pulping process (a potentially cheap and efficient process yielding dissolving grade cellulose having a DP of 400 or less) which is generally unsuitable for use in existing solvent-spun fiber processes, may be used herein according to the teachings of the present invention. For purposes of the present invention, "dissolving grade" cellulose comprises substantially lignin-free cellulose.
In addition to the above, the ability to use low DP cellulose in the process of the present invention is further advantageous because low DP cellulose is theoretically both cheap and abundant, potentially derivable from both municipal and agricultural wastes such as used paper, corn stalk, and sugar cane bagasse.
As a further advantage, the process of the present invention may be employed (with only slight modifications described infra) to form cellulose fibers and films suitable for use in food and pharmaceutical applications. In such instances, food-grade cellulose and food-grade ZnCl.sub.2 are taught for use herein.
In addition to the DP of the cellulose starting material being determinative of the strength of the cellulose fibers of the existing solvent-spun processes, it has also been observed that fiber strength is dependent upon the arrangement of the cellulose crystals within the fiber as well.
To clarify, cellulose may exist in amorphous or crystalline form. In fact, both amorphous and crystalline regions form within the cellulose fiber upon coagulation. The ratio and orientation of these regions vary, but in the existing solvent-spun processes both are determined during the coagulation step.
By way of example, when cellulose is coagulated in a typical solvent-spun process, some molecules randomly orient themselves in a crystalline matrix, the degree of crystallization being determined, generally, by the presence and amount of water in the coagulation medium. Neither the ratio nor the orientation of the crystalline regions can be controlled in such a process, because the crystallization occurs simultaneously with the coagulation of the fiber.
In view this, it was theorized by the present inventor that by separating the steps of coagulation and crystallization in a solvent-spun process, the ratio of crystalline and amorphous regions in the fiber could be controlled. Moreover, by applying tension to the fiber after coagulation but before crystallization, the fiber could be stretched thereby orienting the amorphous and crystalline regions therein. This would result in a solvent-spun cellulose fiber having high tensile strength.
Based on this theory, and in view of the fact that ZnCl.sub.2 was known to be a good cellulose solvent, a process for producing high tensile strength, solvent-spun cellulose fiber was developed.
Additionally, a second embodiment of the present process was found to produce a low tensile strength cellulose fiber particularly suitable for food and pharmaceutical applications when food-grade starting materials and reagents were employed.